1. Field of the Invention
The present invention relates to an image forming device in which printing can be conducted on both sides of paper sheets, more particularly to an image forming device in which erroneous printing on a sheet of paper rather than the rear surface of the desired paper during manual double-side printing can be prevented.
2. Description of the Related Art
Double-side printing on paper has been conducted in image forming devices such as printers and copiers with the object of saving paper. Such double-side printing in image forming devices is conducted in the so-called automatic double-side printing mode, in which handling of paper sheets is conducted automatically, and the so-called manual double-side printing mode, in which a paper sheet that was printed on one side is manually reset in the image forming device. In the automatic double-side printing, the paper sheets have to be turned over mechanically and a mechanism designed therefor is provided. Furthermore, because the paper transportation path is extended, a mechanism for oblique passage correction is also required. Therefore, though such a system has a merit of automation, the demerits thereof include enlarged size, complex mechanisms, and increased cost. On the other hand, the manual double-side printing does not require special mechanisms, but the printing obviously has to involve manual operations. Furthermore, the user has to be careful to avoid confusing the direction of paper sheets which are set during printing.
Accordingly various suggestions have been made to improve the manual double-side printing. For example, Japanese Patent Application Laid-open No. 2000-289259 describes a technology comprising the steps of determining the orientation of paper sheets by detecting the discrimination marks, which were recorded on the paper sheets, during double side printing and executing the double-side printing after setting the appropriate image orientation.
On the other hand, some image forming devices such as printers have a plurality of paper feed means and in which paper sheets of a plurality of types are set at the same time, in order to avoid replacing a paper feed cassette and setting the paper sheets on the paper feed tray, when printing is conducted on paper of different types and sizes. In such devices, a paper feed unit having set therein the paper sheets matching the type of paper sheets designated by an application or printer driver is automatically selected and printing is executed by feeding the paper therefrom, unless a special designation is provided during printing. In such a case, when no paper is present in the selected paper feed unit, the image forming device is most often controlled so as to select a paper feed unit based on a priority order of the paper feed units set in the image forming device.
In the above-described conventional image forming devices having a plurality of paper feed units, no special consideration is usually given to controlling the feed unit from which paper is fed to the above-described manual double-side printing. Therefore, there is a risk that paper feed will be automatically started from a paper feed unit where paper sheets are present prior to setting manually the paper sheet that is necessary during rear-side printing (second pass printing) of the manual double side printing and that the data to be printed on the rear side will be printed on a wrong paper sheet.
Furthermore when the paper sheets are consumed in the course of rear-side (second pass) printing, for example, in the case where all the paper sheets cannot be set at once into a paper feed tray when large volume printing is carried out in a manual double-side printing mode, there is also a risk that paper will be automatically fed from another paper feed unit to continue printing on the rear side before the paper remaining after the front-side printing is replenished. If such wrong paper is fed, the so-called paper jam can sometimes occur when paper sheets of different type are fed.